Joe writes: “I just came across some steaks in the freezer and parts of them look as if they’ve already been cooked. They were definitely raw when I put them in there! Is that what they call “freezer burn”? Are these steaks still safe to eat?”

Yup, it sounds like you’ve got some freezer-burned steaks on your hands, Joe. And yes, they’re still perfectly safe to eat—assuming, of course, that they were safe when you put them in the freezer and that you haven’t had any power outages that caused things to thaw.

But the parts that look cooked probably won’t taste very good once those steaks have been thawed and then cooked for real. I’d suggest thawing the steaks and cutting the burned parts away before cooking them.

What is Freezer Burn?

On the face of it, freezer burn seems sort of mysterious. How can raw food get cooked by sitting in the freezer? Despite appearances, however, Joe’s steaks have not been partially cooked. To explain what’s going on here, I turned to Sabrina Stierwalt, host of the Everyday Einstein podcast. Can you explain the science behind freezer burn?

Everyday Einstein: Freezer burn is the dehydration or removal of water from our frozen foods. When the water molecules in food are frozen into ice crystals, those ice crystals will leave the food’s surface in favor of the dry freezer air if the food is exposed to air. The ice leaves via a process called sublimation—the molecules go directly from a solid (the ice) to a gas (water vapor). So if you missed a spot in your saran wrap coverage or you didn’t get that lid on the ice cream all the way, those foods will be more vulnerable to freezer burn.

Fluctuating temperatures in your freezer can also lead to freezer burn in even the best wrapped foods. Warmer temperatures will cause the ice crystals to melt, and if that water drips off the surface of the food, dehydration takes place.

Nutrition Diva: So, obviously, having a package with a lot of air in it (or packaging that doesn’t keep the air OUT) might increase the risk of freezer burn. But does the water content of foods come into play? Vegetables are very high in water, for example, but meat is relatively low in water. Are foods that are high in water more or less susceptible?

Everyday Einstein: If a food has a higher water content, it has more water to lose before freezer burn makes it inedible. So, a frozen lasagna is going to be more vulnerable than frozen soup—especially if it’s not well wrapped. Similarly, because meat has a lower water content than vegetables, it’s more likely to get freezer burned. Given enough time, the ice crystals will eventually find their way out. So, different foods will have different freezer shelf lives.

By minimizing contact with air, we can minimize freezer burn.

How to Prevent Freezer Burn

Now that we understand what causes freezer burn, we can take steps to prevent it.

Wrap foods tightly. By minimizing contact with air, we can minimize the dehydration and oxidation that leads to freezer burn. If you’re storing foods in containers (as opposed to wrapping them in something), match the size of the container to the contents so that you don’t have a lot of air in the container. Remember to leave a little extra room for the contents to expand slightly when they freeze.

Freezing is not forever. The longer foods are in the freezer, the greater the risk of freezer burn, so don’t leave foods in the freezer for too long. Write the date on the package when you put it in the freezer and keep track of your inventory. Try to eat foods within three or four months of freezing them.

Make sure the freezer stays cold.If temperatures are fluctuating above 0 degrees F (-18 degress C) it can speed the movement of water out of the food and into the air in the freezer. Keep a thermometer in the freezer so you can check the internal temperature of the freezer. If it’s too high, you can adjust the setting on your freezer or refrigerator. Try to open and shut the freezer as quickly as possible to minimize fluctuations.

My thanks again to Sabrina Stierwalt of the Everyday Einstein podcast for stopping by. If you have a question for Sabrina about science or a question for me about nutrition, post them below.

You can also reach me through the Nutrition Diva Facebook page but as you probably know, Facebook has recently made some big changes to what is showing up in your newsfeed. If you want to continue to see the nutrition tips, research, links, videos, recipes and Q&A that I post on Facebook, you can edit your Newsfeed Preferences to tell Facebook to include that in your feed. (Here's a quick video with details).